Ron used the Deluminator to magically travel to where Harry and Hermione were camping once they said his name aloud. Ron arrived just in time to rescue Harry from drowning in the icy pond where the Sword of Gryffindor had been placed, and then used the Sword to destroy the locket horcrux. Despite these heroics, Hermione was, well, somewhat cool toward Ron when he and Harry went back to the tent.